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Avoid listeria infections!

03/1999, 17.03.1999

BgVV provides information on hygiene problems and health risks from foods

It is only since the beginning of the 1980s that infections in humans with Listeria monocytogenes can be clearly attributed to the consumption of contaminated food. A wide range of foods, mainly of animal but also of plant origin, was identified as the cause of individual cases or collective outbreaks of the disease. One of the first, larger recorded outbreaks of listeriosis in Canada was caused by lettuce (1981 with 41 cases). Recently, cases of illness in the European Union have been recorded in conjunction with the consumption of different types of soft cheese made from raw milk. The related press reports have caused unease amongst the general population.

Listeria are bacteria which are widespread in the environment above all in agriculture. The species Listeria monocytogenes also plays a role as a pathogen in humans and animals. In humans the disease is normally relatively harmless, similar to an influenza infection. Some risk groups, including pregnant women and newborn babies, may develop serious symptoms (stillbirth or pre-term births and infant listeriosis). In older people and people with a weakened immune response to infection, severe forms of listeriosis may occur with the clinical picture of meningitis or sepsis. 30% of these cases are fatal.

Studies by the health authorities both in Germany and in other EU Member States have revealed that the most diverse foods may contain pathogenic listeria. By way of example, here are some study results from the control authorities of the federal Länder. They were obtained in 1991 and 1992 within the framework of official food control. According to these results, 2.2% of the samples of soft cheese, 3.9% of the samples of sour milk cheese, 4.1% of the samples of sheep and goat cheese, 6.9% of the samples of sausage for heating in simmering water, 14% of ham samples (vacuum-packed), 5.9% of fish product samples (unpackaged 3.5%, vacuum-packed 8.5%) and 4.8% of pasta samples were contaminated with listeria. Listeria monocytogenes was also detected in fresh vegetables, particularly in prepared salads.

Despite the relatively widespread presence of listeria in foods, the number of cases of disease in Germany is relatively low. Newborn listeriosis is a notifiable disease and the number of cases ranges between 30 and 40. From the data a total number of listeriosis cases for all population groups can be estimated of around 200 individual cases. The main reasons for this low infection rate is thought to be the low content of listeria in foods (under 100 listeria per gram).

Food may become contaminated with listeria at various stages in production and processing. Foods of animal origin in particular like raw milk and raw meat may become contaminated during collection, e.g. during milking or slaughter.

Therefore, it cannot be ruled out that in cheeses manufactured from raw milk, contamination of the starting milk is the cause for the occurrence of listeria in the final product. In the case of cheese made from heat-treated milk, listeria are normally killed when milk is correctly pasteurised. Poor hygiene in the processing stage may open the door to the risk of new contamination of the product after heat treatment. This applies not only to cheese but also to other foods of animal and plant origin. Examples of poor hygiene are dirty working surfaces and appliances in the food plants, poor or too infrequently cleaned slicing machines in the food trade and shortcomings in the personal hygiene of staff.

The ability of listeria to survive and multiply in food depends on the manufacturing methods and technological treatment. Boiling, roasting, sterilising and pasteurising all kill the bacteria. In foods which contain too little water, too much salt or preservatives or are very acidic (e.g. sauerkraut, mixed pickles and yoghurt), multiplication is delayed or not possible at all. Listeria have good growth opportunities compared to competitive germs where there is a reduced oxygen supply (e.g. in vacuum-packed sausages for heating in simmering water and smoked fish) and long storage times of the food under refrigerated conditions.

For the above reasons it is likely that many foods contain listeria. As a consequence, infections in humans cannot be ruled out according to the current level of scientific knowledge. The infection risk can however be reduced if consumers, in particular people with a weakened immune system,

• thoroughly cook meat and fish dishes,
• boil raw milk and
• do not eat minced meat raw.

In addition, pregnant women should not eat raw milk soft cheese and should not eat cheese rind (of other types of cheese as well).

Furthermore, BgVV recommends strict compliance with the minimum shelf life date in the case of perishable foods and, where possible, consumption of vacuum-packed products long before this date expires.

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